As technology for producing small, portable electronic devices progresses, so does the need for electronic displays which are small, provide good resolution, and consume small amounts of power in order to provide extended battery operation. Past displays have been constructed based upon cathode ray tube ("CRT") or liquid crystal display ("LCD") technology. However, neither of these technologies is perfectly suited to the demands of current electronic devices.
CRT's have excellent display characteristics, such as color, brightness, contrast and resolution. However, they are also large, bulky and consume power at rates which are incompatible with extended battery operation of current portable computers.
LCD displays consume relatively little power and are small in size. However, by comparison with CRT technology they provide poor contrast, and only limited ranges of viewing angles are possible. Further, color versions of LCDs tend to consume power at a rate which is incompatible with extended battery operation.
At least partially as a result of the above described deficiencies of CRT and LCD technology, efforts are underway to develop new types of electronic displays for the latest electronic devices. One technology currently being developed is known as "field emission display technology." The basic construction of a typical field emission display, or ("FED"), is shown in FIG. 1. As seen in FIG. 1, field emission display 10 comprises an anode, generally designated 20, a cathode, generally designed 30, and a plurality of spacers 40 which prevent the anode 20 and cathode 30 from being pushed into contact with each other by exterior atmospheric pressure when the space between the anode and cathode is evacuated.
The anode 20 typically comprises a flat glass plate 101 with a transparent conductor layer 102 formed on its lower surface. The screen area of the anode (designated 104 in FIG. 2) includes a large number of phosphor dots 112 formed on the lower surface of transparent conductor 102.
Cathode 30 comprises a substrate or baseplate 114 on which thin conductive row electrodes 108 are formed. Silicon baseplate 114 may be single crystal silicon. The row electrodes may be formed from doped polycrystalline silicon that is deposited on the baseplate and serves as the emitter electrode, and are typically deposited in strips that are electrically connected. A resistive layer (not shown) may be deposited on top of the row electrodes 108 and spaced-apart cathode emitters 106 are in turn formed on top of the row electrodes 108. Also formed on the row electrodes 108 and baseplate 114 is a dielectric layer 116 on which, in turn, is a conductive layer 110 which forms a gate electrode and controls the emission of electrons 107 from emitters 106. Typically, millions of emitters 106 are required to provide a spatially uniform source of electrons.
FIG. 2 is a exploded diagram of an FED package, showing the anode 20 and cathode 30 of FIG. 1, together with additional components (e.g., a getter 35, a seal frit 40, backplate seal ring 45, frit layer 50, and backplate 55 with a compressible dot 60) that are typically included in the complete FED package. As is apparent from FIG. 2, it is important that the various components of the FED package, particularly the anode 20 and cathode 30, be positioned accurately relative to each other.
Conductors on a spacer ring 22 on anode 20 are bonded to conductive leads on the cathode 30, and the cathode and anode must be precisely positioned to each other at the time this bond is made. One method of connecting the conductive leads on the cathode 30 to the conductors on the spacer ring 22 of anode 20 is commonly known as flip chip bonding. In flip chip bonding, contact pads (not shown) on one substrate, e.g., on cathode 30, are provided with conductive "bumps" which are carefully aligned with the conductors on the spacer rail of another substrate (e.g., of anode 20). An apparatus commonly referred to as a flip chip bonder then bonds the contact pads of the cathode to the conductors of the anode using a process commonly referred to as thermo-compression bonding. Although the following discussion is directed to a procedure using a flip chip bonding machine or bonder used to bond the anode 20 and cathode 30 of an FED, it will be understood that procedures employing other types of bonders or the bonding of substrates of different devices are equally applicable.
Regardless of the particular bonder or procedure employed, the alignment between the cathode 30 and anode 20 is critical to obtaining a properly functioning FED. Accordingly, many flip chip bonders and the like are provided with some type of "machine vision" system which automatically aligns the cathode and anode prior to bonding.
However, machine vision alignment systems are not sufficiently accurate to ensure completely acceptable alignment. Therefore, after various portions of a device have been bonded together, the device is removed from the production line and taken to a test station at which a test procedure, commonly referred to as "veneering," is performed to evaluate the accuracy of the alignment.
FIG. 3 shows exemplary aligning marks 301, 305, (commonly referred to as "fiducials") on a pair of substrates 300 and 304 (which may be, for example, an anode assembly 20 and cathode assembly 30). In the prior art systems and in the practice of the present invention, aligning marks 301, 305 are used by a "machine vision" system to align the two substrates just prior to the two substrates being bonded together.
FIG. 3 also shows exemplary veneering marks 302, 306 on, respectively, substrates 300 and 304, for use in post-bonding inspection and evaluation. As will be apparent, aligning marks 301, 305 are provided adjacent two diagonally opposite corners of the substrates, while veneering marks 302, 306 are provided along pairs of adjacent edges.
Each aligning mark 305 on substrate 304 (e.g., on an anode assembly 20) is an open circle or "doughnut", typically having an inner diameter of about 100 microns and an outer diameter of about 200 microns. Each aligning mark 301 on substrate 300 (e.g., on a cathode assembly 30) is a solid round dot about 50 microns in diameter.
According to prior art practice, a "machine vision" system (e.g., a so-called "look-up, look-down" imaging system of the type used conventional flip bonders (those sold by Sierra Research and Technology, Inc. of Westford, Mass., Micro Robotics Systems, Inc. of Chelmsford, Mass. and RD Automation of Piscataway, N.J.) are used to automatically to align the two substrates to be bonded together so that each solid dot 301 on substrate 300 is centered within a respective round doughnut 305 on substrate 304. The machine vision system views the alignment marks on the two substrates and, either using pattern recognition software or by projecting images of the substrates on a video screen where they may be viewed by an operator, achieves alignment of the two substrates so that each solid dot 301 on substrate 300 is centered within a respective round doughnut 305 on substrate 300. This alignment is achieved with the two substrates in close proximity to each other, and the only additional movement required to bring them into contact for bonding is in the z-direction.
According to the prior practice, after the two substrates have been bonded together, the workpiece is removed from the production line and taken to a test station at which the veneering marks 302 and 306 are employed to evaluate the accuracy of the alignment of the substrates, e.g., of the anode assembly 20 and cathode assembly 30, under a microscope. Desirably, the two substrates will be aligned so that each veneering mark 302 on substrate 300 will be centered within a respective veneering mark 306 on the other substrate 304.
The configurations of veneering marks 302 and 306 are most clearly shown in FIGS. 4 and 5.
As shown in FIG. 4, each veneering mark 306 on substrate 304 comprises a row of identical boxes 401, equally spaced from each other. In the illustrated embodiment, each veneering mark 306 includes twenty aligned boxes 401.
Referring now to FIG. 5, each veneering mark 302 on substrate 300 includes a row of axially aligned bars 501. Each bar 501 is exactly the same length, a length equal to distance between a pair of adjacent boxes 401 so that each bar is capable of fitting precisely in the interval between a pair of adjacent boxes. The center-to-center spacing of bars 501, however, is slightly different than (in the illustrated embodiment 0.5 microns greater than) the center-to-center spacing of boxes 401; and the total number of bars 501 in each veneering mark 302 (in the illustrated embodiment twenty-one bars) is typically different (in the illustrated embodiment one greater than) from the number of boxes 401 in the corresponding veneering mark 306. A pair of arrows 502 are provided on the opposite sides of the center bar 501a, with the heads of the arrows pointing towards each other.
FIGS. 6, 6A and 7 illustrate the relative positioning of superposed veneering marks 302, 306 when the two substrates are (FIGS. 6 and 6A) or are not (FIG. 7) precisely and accurately aligned relative to each other. In each of FIGS. 6 and 7, the two arrows 502 of veneering mark 302 are positioned in the two center boxes 401a, 401b of mark 306.
In FIGS. 6 and 6A, in which the two substrates are aligned, the ends of the center bar 501a of mark 302 are tangent to the adjacent edges of boxes 401a and 401b. In FIG. 7, in which the two substrates are not perfectly aligned, center bar 501a is offset slightly, so that its left end is spaced slightly from the adjacent edge of box 401a and its right end projects slightly into box 401b. However, it will also be noted that the ends of another bar, i.e., bar 501g, appear to be tangent to the sides of the two boxes 401f and 401g between which the bar is positioned. Thus, the extent of the misalignment of the marks 302 and 306 in FIG. 7 can be accurately be determined simply by counting the number of boxes 401 between the bar 501a between the two center boxes 401a, 401b, and the apparently aligned bar 501g. In FIG. 7, there are six such boxes, and the extent of the misalignment is accordingly six (6) times the difference (0.5 microns) between the center to center spacings of the bars and boxes, or 3.0 microns.
It will thus be appreciated, that existing "veneering" procedures make it possible vary accurately to evaluate the extent of misalignment between two components that have been bonded together. Unfortunately, in order to evaluate alignment using such existing procedures, the device being evaluated must be taken out of the production line for evaluation, and then returned to the production line when the evaluation is complete. This results in production delays and additional product handling. There is a need for a system which evaluates and improves the quality of the alignment between the die and the substrate on-line.